In recent years, with the increasing use of various electric installations and electronic installations, electromagnetic interference (EMI) has been rapidly increased. It has been indicated that EMI causes malfunction or troubles of electronic and electric appliances and further gives disorders to operators of these appliances. Accordingly, for electronic and electric appliances, it has been required to control the intensity of released electromagnetic wave within standards or regulations.
To cope with EMI, there is a need to shield an electromagnetic wave. For this purpose, it is self-evident that a property of a metal that does not pass the electromagnetic wave may be used. For example, a method in which a case is formed with a metallic product or a highly conductive product, a method in which a metallic plate is inserted between circuit substrates, a method in which a cable is covered with a metallic foil, and the like have been employed. However, since an operator has to recognize characters and the like displayed on a screen in CRT, PDP and the like, a transparency is required for a display. Accordingly, the foregoing methods have been inappropriate as a method for shielding an electromagnetic wave because the display surface is often non-transparent.
Especially, PDP generates large quantities of an electromagnetic wave in comparison with CRT and the like, so that a stronger electromagnetic wave shieldability is required. An electromagnetic wave shieldability can simply be expressed in terms of a surface resistance value. For example, in a light-transmitting electromagnetic wave shielding material for CRT, the surface resistance value is required to be approximately 300 Ω/sq or less, whereas in a light-transmitting electromagnetic wave shielding material for PDP, it is required to be 2.5 Ω/sq or less. In a plasma television for people's livelihood using PDP, it is highly required to be 1.5 Ω/sq or less, more preferably 0.1 Ω/sq or less. Thus, quite a high conductivity is required.
Regarding a level required for a transparency, a transmittance in the entire visible region is required to be approximately 70% or more for CRT and 80% or more for PDP. Thus, a still higher transparency is desired.
For solving the problems, various materials and methods have been so far proposed in which both of the electromagnetic wave shielding property and the transparency are satisfied using a metallic mesh having an opening portion, as will be described below.
(1) Mesh with a Silver Paste Printed
For example, a method in which a paste made of a silver powder is printed in a network state to obtain a silver mesh is disclosed (refer to, for example, gazette of JP-A-2000-13088). The silver mesh obtained by this method is problematic in that a line width is large to decrease a transmittance because of a printing method, or the like. Further, a surface resistance value is large and an electromagnetic wave shieldability is low. For this reason, there was a need to plate the resulting silver mesh for increasing the electromagnetic wave shieldability.
(2) Irregular Network-Like Silver Mesh
For example, an irregular micro-network-like silver mesh and a method for producing the same are disclosed (refer to, for example, gazette of JP-A-10-340629). However, this method is problematic in that only a mesh with a large surface resistance value of 10 Ω/sq (low electromagnetic wave shieldability) is obtained. Further, it is problematic in that a haze is as large as more than 10-odd % to dim a display image.
(3) Etched Copper Mesh Using Photolithography
A method in which a copper foil is etched using photolithography to form a copper mesh on a transparent substrate is proposed (refer to, for example, gazette of JP-A-10-41682). This method is advantageous in that since fine processing of a mesh is possible, a mesh with a high opening rate (high transmittance) can be produced and strong electromagnetic wave release can also be blocked. However, the method is problematic in that it includes quite a large number of steps and production has to be conducted through these steps.
Further, since the copper foil is used, the final mesh shows not a black color but a color of the copper foil to cause the decrease in contrast of an image in a display device. Still further, owing to the etching method, there is a problem that an intersecting portion of a lattice pattern is thicker than a line of a linear portion and its improvement is required in relation to a problem of moire.
(4) Method for Forming Conductive Silver Using a Silver Salt
In the 1960s, a method in which a metallic silver thin film pattern having a conductivity is formed by a silver salt diffusion transfer method for depositing silver on a physical development nucleus has been disclosed (refer to, for example, gazette of JP-B-42-23746).
However, an approach in which an electromagnetic wave released from an image display surface of a display of CRT, PDP or the like is shielded by making a conductive metallic silver thin film through this method without disturbing image display was entirely unknown.
According to this method, a silver thin film of 10 Ω/sq to 100 Ω/sq is obtained, but this level of the conductivity is insufficient for use in PDP. Moreover, a transparency is also insufficient, so that there is a problem that the transparency and the conductivity cannot be satisfied.
Accordingly, even though the silver salt diffusion transfer method is used as such, the light-transmitting electromagnetic wave shielding material excellent in light transmittance and conductivity and advantageous for shielding the electromagnetic wave released from the image display surface in the electronic display device could not have been obtained.